Water Resources--Office of Water Quality
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By D.B. Radtke, F.D. Wilde, M.W. Sandstrom, and K.K. Fitzgerald
Water samples must be processed as quickly as possible after collection. The equipment most commonly used for sample processing includes sample splitters, filtration units or assemblies, solid-phase extraction systems, and chambers in which samples are processed and treated with chemical preservatives. Having several available sets of cleaned processing equipment is recommended. The equipment and supplies used to process surface-water and ground-water samples are listed in section 2.4.
The collection of surface water generally results in a single composite sample. A ground-water sample generally is not composited; instead the sample is pumped directly into separate bottles for designated analyses. There are exceptions. For example, a ground-water sample can be composited when the sample is collected using a nonpumping method (bailer or thief sampler).
Once a sample has been composited, the sample is often subdivided (split) into subsamples for analysis. Each whole-water subsample should contain suspended and dissolved concentrations of target analytes that are virtually equal to those in every other subsample. Uses of the churn splitter (fig. 2-9) and the cone splitter (fig. 2-10) are discussed in sections 2.2.1.A and 2.2.1.B, respectively, and testing and comparative data between the splitters are described in Capel and others (1995, 1996) and in Office of Water Quality Technical Memorandum 97.06.
The 8 or 14-L plastic churn splitter is recommended to composite and split surface-water samples for trace-element analysis (fig. 2-9). Stainless steel and glass containers are used to composite samples for organic analysis. To avoid sample contamination, do not collect or extract samples for trace-element analyses from a metal container, or samples for organic-compound analysis from a nonflurocarbon-polymer plastic container.
The following modifications to the churn splitter and its deployment are required:
|Modified churn spigot: This spigot is described in Horowitz and others (1994) and is available from the USGS Quality of Water Service Unit (QWSU) in Ocala, Fla.|
|-||The spigot contains a metal spring to keep the spigot valve closed when not in use. If the spigot leaks, sample can contact the spring. The spring represents a potential source of metal contamination.|
|-||To prevent leakage of the spigot, silicone sealant is injected inside the push-button mechanism.|
|Funnel assembly: To meet requirements for trace-element sampling, a funnel assembly is inserted into a 1-in. hole drilled through the lid of the churn splitter (fig. 2-9).|
|-||The funnel is used when pouring whole-water samples into the churn splitter so that the churn lid can be left on, thus minimizing exposure of the composite sample to atmospheric contamination.|
|-||To make the funnel assembly, cut the top section (at the shoulder line) from a 1-L polypropylene sample bottle and insert into the hole drilled in the churn lid. Cut the bottom two-thirds from of a 1-L NALGENEtmor other larger diameter sampler bottle and use as a funnel cap.|
|Churn covering: To keep the entire churn-splitter assembly clean during sampling and prevent potential contamination, the churn splitter is placed inside two pliable, clear plastic bags (double bagged). These bags should be large enough to completely enclose the churn splitter, including funnel and churn handle, with enough excess material so that the bag openings can be gathered, folded over, and kept closed.|
|Churn carrier: The carrier is a white plastic container, with lid, large enough to hold the double-bagged churn splitter assembly.|
|The lid of the carrier serves both as a seal for the carrier and as a windbreak when the collected subsample is poured into the churn funnel.|
|-||The purpose of the carrier is to minimize contamination during transport. In exposed areas, such as bridges and roadways, the carrier can protect against atmospheric sources of contamination, particularly material from motor vehicles.|
Volume of sample needed:
|Subsamples totaling 10 L can be withdrawn from the
Advantages of the churn splitter:
|Simple to operate.|
|Easy to clean.|
Limitations of the churn splitter (see also Ward and Harr (1990), Horowitz and others (1994), and Capel and Larson (1996), and Office of Water Quality Technical Memorandum 97.06):
|Although it can be used to split samples with particle sizes
|Sample volumes less than
|Plastic (nonfluorocarbon polymer) churn splitters should not be used to composite samples for determination of organic compounds.|
|Samples for bacteria determinations are not to be taken from a churn splitter because the splitter cannot be autoclaved.|
The cone splitter is a pour-through device constructed entirely of fluorocarbon
polymers (fig. 2-10). The cone splitter may be used
to process samples with particle sizes
The cone splitter is built to very close tolerances in order to achieve accurate and reliable operation. Bias to data can result from splitter imperfections or improper operation; therefore, calibration and proper use is necessary when processing samples.
Advantages of the cone splitter
|Used to process samples with suspended-sediment concentrations from 1,000
|Samples as small as
|Samples greater than
|Samples to be analyzed for organic compounds (except for volatile organic compounds) can be processed through the fluorocarbon polymer cone splitter.|
|Samples containing sediment particles ranging in size from very fine clay
and silt (1 to
Limitations of the cone splitter
|Accuracy of the volume equivalents must be verified before using a new or modified cone splitter (see instructions below).|
|Splitter is awkward to operate and clean in the field.|
|Sample is vulnerable to contamination from atmospheric sources or from improper operation.|
|Splitting capability for sediment particles
|Samples for bacteria determinations are not to be collected with the cone splitter because the splitter cannot be adequately sterilized.|
|The cone splitter must be level for proper operation.|
Before using a new or modified cone splitter, test the splitter to be used as follows (Office of Water Quality Technical Memorandum 80.17):
Processing and preservation chambers reduce the possibility of random atmospheric contamination during sample splitting, filtration, and preservation. These chambers are required for samples for trace-element determinations (Horowitz and others, 1994). The processing chamber can serve also as a collection chamber for pumped samples. There is no standard design for either fixed or portable chambers; however, to prevent contamination of inorganic samples with metals, the materials used in their construction should be either nonmetallic or completely covered by or embedded in nonmetallic material. Plastic components have been tested and do not emit volatile substances that might contaminate a VOC sample. This can be further documented by collecting an ambient blank in the chamber(s).
Fixed chambers can be enclosures permanently installed in a field vehicle for the sole purpose of sample collection, processing, or preservation. Fixed chambers must not be used as a storage area. The portable chamber illustrated is inexpensive and easily constructed with 1/2-in. white polyvinyl chloride (PVC), tubes which are used to support a clear plastic bag (fig. 2-11). The transparent bag forms a protective tent to work within while collecting, processing, or preserving samples. Another option is to purchase or construct a fixed or portable glove box. The glove box also should have no exposed metal parts. Glove boxes that can be filled with inert gas should be used for samples to be excluded from contact with atmospheric gases.
|The processing chamber sits over a fixed or portable sink lined with plastic to which a waste-disposal funnel has been attached, allowing rinse water to drain to waste. If a sink is not available, a waste bottle is placed inside the chamber to contain liquid waste and overflow. A hole can be drilled through the top crossbar of the processing chamber through which sample tubing can be inserted and a filter assembly attached.|
The preservation chamber consists only of the frame and bag cover. A uitable container placed inside the preservation chamber is needed to contain spent preservative ampoules (see NFM 5).
Use of separate chambers for sample processing and sample preservation can be the most convenient approach and is necessary if more than one type of chemical preservation will be used. Multiple preservation chambers can help prevent cross contamination and also save time by eliminating the need to change covers between treatments. However, when only a single preservative is needed (such as nitric acid), it can be added while the sample is in the processing chamber without a bag change, after all other samples have been removed.
Where space inside the field vehicle is extremely limited, having both a processing chamber and a preservation chamber set up at the same time might not be feasible. Under such circumstances, the processing-chamber frame may also function as a preservation-chamber frame as long as the cover is changed before sample preservation and is clipped to the chamber frame rather than supported over the frame. The preservation chamber cover must be changed every time the preservation procedure requires a change in gloves (NFM 5).
Filtration separates particulate substances (solid-phase and biological materials) from the solute or aqueous phase of a water sample. Water samples are filtered for analysis of inorganic constituents, organic compounds, and biological materials to help determine the environmental fate and quantify the transport of these target analytes. Detailed information about filtration can be found in Kennedy and others (1976), Ward and Harr (1990), Horowitz and others (1994), and Koterba and others (1995).
For surface water, the most common filtration system consists of a reversible, variable-speed battery-operated peristaltic pump or a metering pump that forces the whole-water sample through tubing into a filter assembly. For ground water, the sample ordinarily is pumped through a sample line directly into a filter assembly. If the sample is collected by bailer, the sample generally is emptied through a valve with fluorocarbon polymer tubing and is transferred to the filter assembly by means of a peristaltic pump. Some bailers can be directly fitted with a filter and hand-pump system.
TECHNICAL NOTE: Separation of solid from aqueous phases can be achieved by methods other than filtration, and data requirements may dictate the need for an alternative method such as centrifugation, ultracentrifugation, dialysis or lipid-membrane separation, and reverse-flow osmosis and tangential-flow filtration.
The filter assembly to be used depends on the type of target analytes, which are discussed in the following sections. Membrane filters commonly used to filter inorganic samples generally are made of cellulose nitrate, polycarbonate polymers, or polyethersulfone-based media. These filter media are not suitable for filtering samples to be analyzed for organics; glass microfiber is the media used for filtering most organic samples (silver filters are used for dissolved-organic compound samples).
A filtered sample is defined operationally by the nominal pore size of the filter media used.
|The filter pore size selected depends on study objectives, data requirements, and industry standards.|
|The standard pore sizes of filter media used by the USGS are|
|--||0.7 µm for pesticides, most other organic compounds, and some bacteria (NFM 7).|
|--||0.45 µm for inorganic constituents (including major ions, radiochemicals, and trace elements), some bacteria (NFM 7), and dissolved organic carbon.|
|See NFM 7 for a description of filter media for biological analysis.|
Samples for analyses of inorganic constituents are filtered by use of either a disposable capsule-filter assembly or a plate-filter assembly (table 2-7). Construction materials of filtering systems must not be a source of sample contamination with respect to the substances for which the sample will be analyzed.
The protocol for filtering a sample for analysis of inorganic constituents (inorganic sample) is to use a disposable capsule filter such as the Gelman 12175 ground-water sampling capsule (fig. 2-12). Use of a disposable capsule filter eliminates the potential for contamination from a reusable filter assembly (such as the plate filter) and also eliminates time-consuming field-cleaning procedures that are required for a reusable assembly after each sample is filtered.
Horowitz and others (1994) concluded that the disposable capsule filter would
not be a probable source of sample contamination for inorganic constituents,
including trace elements, major ions, nutrients, stable isotopes, and radiochemicals
(table 2-7) if precleaned with a minimum of
Advantages of the disposable capsule filter compared to the plate-type filter:
|Capsule filters are sealed units; hence, the likelihood of contamination is reduced because the filter itself is not handled.|
|Surface area of the capsule filter is roughly three times that of the 142-mm plate filter and is less subject to clogging.|
|Cleaning the filter between samples is not necessary because each capsule filter is used only once and then discarded.|
|QWSU provides quality-control checks of each lot of capsule filters and provides a certificate of analysis with each filter.|
Do not use the disposable polyethersulfone capsule filter for organic samples.
Before 1994, the most common filtration assembly used for USGS studies for filtering inorganic samples was the nonmetallic backflushing plate-filter assembly designed to hold a 142-mm filter. Two screens support the filter media (one above and one below) and allow water flow in either direction without disruption of the membrane. A smooth-tipped plastic forceps is needed to transfer the filter media to the plate of the filter assembly. (Kennedy and others, 1976, give a detailed description of and instructions for use of the plate-filter assembly.)
|Types of plate-filter assemblies for inorganic samples include: -- Plastic backflushing assembly (described above), available for 47-mm, 142-mm, and 293-mm filters (fig. 2-13).|
|--||Plastic vacuum filter assembly for 47-mm-diameter filter; used with either a hand vacuum pump or a peristaltic pump.|
|--||Fluorocarbon polymer filter assembly designed for 47-mm-diameter filters; can be used for in-line filtering of inorganic or organic samples by changing to the appropriate filter media.|
|The plate-filter assembly is no longer recommended for routine filtration of samples for analysis of inorganic constituents, but is still used to filter samples for analysis of major ions, nutrients, stable isotopes, and radiochemicals (table 2-7).|
Filtering whole-water samples isolates suspended solid-phase substances from the aqueous phase, thus allowing separate determinations of organic compounds in each phase.
As preparation for determining organic compounds that partition onto suspended matter (hydrophobic compounds)--Filtering primarily concentrates suspended materials on the filter, enhancing extraction efficiency and lowering analytical detection limits. This is especially useful for whole-water samples with small concentrations of suspended material where large volumes of sample (4 to 40 L) must be filtered to provide an analyzable mass of suspended materials.
As preparation for determining organic compounds that are more water soluble (hydrophilic)--Filtering is used to remove suspended material, because it often contains interfering constituents that are co-extracted with target analytes.
Filtering also helps to preserve samples for organic determinations because microorganisms that could degrade compounds in the sample are removed (Ogawa and others, 1981).
Equipment needed to filter samples for determination of organic compounds is described in detail in Sandstrom (1995) and includes a positive displacement pump, an aluminum plate-filter assembly (different filter assemblies can be required, depending on the analysis to be done), the filter media, and metal forceps. All equipment and components used for filtering whole-water samples for organic determinations should be made of materials that will not contaminate the sample or sorb analytes and that are suitable for use with organic cleaning solvents. Such materials include stainless steel or aluminum, fluorocarbon polymer, glass, and nonporous ceramics (hard-fused microcrystalline alumina). Other materials can be used, but they must not introduce contaminants or cause sorptive losses, and they must be sufficiently resistant to degradation by cleaning solvents. Use of plastics, rubber, oils and other lubricants are to be avoided because they can result in sample contamination, analytical interference, and (or) sorptive losses.
The valveless piston metering pump consists of a pump head with a reciprocating piston driven by a 12-volt direct current (DC), variable-speed motor (fig. 2-14). It has a delivery rate of up to 500 mL per minute.
The pump head and all wetted parts are constructed of ceramic, fluorocarbon polymer, or stainless steel components, which are resistant to organic solvents. These pumps can tolerate some suspended materials in the sample being pumped, but large concentrations of suspended materials can cause excessive wear of pump parts and strain on the pump motor.
The ceramic piston and shaft of these pumps will break if motor amperage exceeds 4 amps. To avoid this, either a 4-amp DC circuit breaker should be used in-line with the pump power line or an alternating to direct current converter with 4-amp maximum output should be used.
The pump and filter assembly are connected by 1/4-in.-diameter convoluted fluorocarbon polymer tubing with appropriate fittings. The convoluted tubing does not crimp when bent, unlike straight fluorocarbon polymer tubing.
The filtration equipment described in the following list includes various types of plate-filter assemblies, one of which is used for solid-phase extraction.
|The most common plate-filter assembly consists of two machined aluminum or stainless steel plates, designed to hold a 142-mm-diameter filter, which are held together by locking bolts or a locking ring (fig. 2-15).|
|--||The plates have fluorocarbon polymer-coated silicone or VitonTM O-rings set in grooves to seal the filter assembly. A stainless steel screen on the lower plate supports the filter.|
|--||A valve is built into the upper plate to exhaust trapped air. Connectors are built into the center of the top and bottom plates so that inlet and outlet fluorocarbon polymer tubing can be attached.|
|--||Plate-filter assemblies are available for filters with diameters ranging from 13 to 293 mm. The size selected is determined by the sample volume to be filtered and the concentration of suspended materials in the sample.|
|In-line filter assemblies used specifically for organonitrogen sample filtering include 13-mm stainless steel or 25-mm disposable nylon membrane filter assemblies. The filter assembly is connected to the pumping system by means of appropriate tubing connectors. Further details are given in NFM 5 and in Sandstrom and others (1994).|
|For solid-phase extraction (SPE), a special disposable in-line plastic filter capsule is used to filter organonitrogen herbicide samples before sending the filtrate to the NWQL.|
|--||Although filter assemblies made from plastic components should not be used to process water-sediment samples for organic compound determination, they have been approved for a specific SPE method.|
|--||The housing of this capsule filter is a thin nylon membrane filter, 25 mm in diameter, with a pore size of 0.45 µm. The capsule contains polypropylene with luer lock inlet and outlet connectors.|
|--||Filter units for organonitrogen herbicides by SPE are available from QWSU.|
Tortuous-path depth filters made of borosilicate glass fibers are used to filter most samples for organic determination because the filter materials are basically inert and can be precleaned with organic solvents or baked at 450°C for at least 2 hours. Depth filters also can process larger fluid volumes than membrane filters without clogging. Membrane filters (cellulose or polycarbonate polymers) commonly used to process samples for determination of nutrients and other inorganic constituents are not suitable for filtering samples for organic determinations, mainly because they are not resistant to organic solvents used to preclean sampling and processing equipment. Membrane filters made of silver metal with a 0.45-µm pore size are resistant to organic solvents, but they become clogged too quickly for filtering the relatively large sample volumes (1 to 5 L) often needed for determination of organic compounds.
For organic samples (except for organonitrogen herbicides and suspended/dissolved organic carbon):
|Use glass-fiber filter with 0.7-µm nominal pore size, baked at 450°C for at least 2 hours.|
|--||Obtain 142-mm diameter filter through QWSU.|
|--||Obtain filters with other diameters (0.7-µm nominal pore size and baked) through NWQL by special request.|
|Use only filters without binders. (Acrylic resin binders can leach and contaminate samples, or might not be completely combusted when baked at 450°C.) Binder-free glass-fiber filters are available in various diameters ranging from 13 to 293 mm.|
A stainless steel or fluorocarbon polymer pressure filter assembly (fig. 2-16) fitted with a 47-mm, 0.45-µm-pore-size silver membrane filter is used to separate dissolved from suspended phases of organic carbon.
|Use a hand-pressure pump, peristaltic pump, or an organic-free nitrogen gas tank fitted with clean tubing to apply pressure to filter dissolved organic carbon (DOC) from suspended organic carbon (SOC) samples (Office of Water Quality Technical Memorandum 78.06, see "Internal Documents").|
|Maintain gas pressure applied to the filter assembly at less than 15 lb/in2.|
|Ensure that the gas is clean by way of gas-purveyor certification or by attaching an in-line 0.2-mm Gelman Acro 50 hydrophobic membrane filter disk. Do not use any other type of filter.|
|--||Because the sample does not contact the in-line gas-filter disk, the disk can be used to filter gas used to process multiple samples, or until the disk clogs or is contaminated.|
|--||Store in-line filter disk in a resealable plastic bag between uses.|
Pump tubing refers to the sample lines used with peristaltic, metering, and submersible pumps. Field personnel are cautioned to evaluate possible artifacts in a sample associated with pump tubing and tubing connections.
|Tubing connectors and connections that contact the sample should be made of inert material, to the extent possible. Stainless steel connections must be the highest grade available (SS 316). If flexible copper, aluminum, or stainless steel tubing is used for CFC sampling, it should be refrigeration grade. Such fittings ordinarily are delivered with coatings of machining lubricants, which must be removed by cleaning before use.|
|--||Greaseless fittings can be ordered. Fluorocarbon polymer, silicone, C-FlexTM pump tubing commonly are used with portable submersible pumps and with peristaltic or metering pumps. Use of a fluorocarbon polymer material generally is recommended because fluorocarbon polymers are relatively inert with respect to many inorganic and organic analytes (table 2-8).|
|Silicone tubing is suitable when sampling for inorganic analytes only, and only after appropriate cleaning (see NFM 3).|
|--||Measurable concentrations of silica (0.09 to 0.24 mg/L) have been detected in blank samples passed through silicone tubing (Horowitz and others, 1994). These concentrations are likely to be significant only if low-ionic-strength water is being sampled.|
|--||Silicone tubing was not designed for use with acids.|
|--||Silicone tubing is gas permeable and very sorptive of organic compounds.|
|C-FlexTM tubing is made from a thermoplastic elastomer and is suitable for use when sampling for all inorganic analytes.|
|--||C-FlexTM is relatively resistant to acid. Acid resistance is a factor because dilute hydrochloric acid is required in cleaning procedures (see NFM 3).|
|--||C-FlexTM is less permeable to gas than silicone tubing.|
|Fluorocarbon polymer tubing is recommended when sampling for most inorganic and organic analytes. Fluorocarbon polymer tubing is available in corrugated, convoluted, and straight-wall configurations (fig. 2-17).|
|--||Fluorocarbon polymer tubing premolded to the shape of the pump head is available.|
|--||Convoluted fluorocarbon polymer tubing is flexible and easy to handle. Attach convoluted tubing to each end of the premolded tubing.|
|--||Fluorocarbon polymer tubing sheathed in another plastic is available at lower cost but is not recommended because the fluorocarbon polymer twists, constricts, and cracks.|
|Polyvinyl chloride (PVC) tubing (TygonTM) is suitable for inorganic samples only, and it must be appropriately cleaned prior to contact with inorganic samples.|
|--||PVC tubing can be washed with dilute acid.|
|--||PVC tubing has the lowest gas permeability of any peristaltic pump tubing.|
|--||For use with a peristaltic pump, PVC tubing has a shorter life than silicone, C-FlexTM, or NorpreneTM.|
|--||PVC tubing may leach plasticizers.|
|NorpreneTM tubing is made from a thermoplastic elastomer (a polypropylene base with USP mineral oil) and is suitable when sampling for inorganic analytes only. It must be appropriately cleaned prior to contact with samples collected for inorganic analysis.|
|--||NorpreneTM tubing can be washed with dilute acid.|
|--||NorpreneTM tubing may leach USP mineral oil.|
|--||For use with a peristaltic pump, NorpreneTM tubing has the longest life of any manufacturer-recommended tubing material.|
|--||The gas permeability of NorpreneTM tubing is lower than that of silicone tubing and greater than that of PVC tubing.|
For filtering ground-water samples that are pumped directly from the well to a filter assembly, C-FlexTM or fluorocarbon polymer tubing is recommended. (Nylon tubing also might be an acceptable alternative, but it has not been tested as a source of trace-element or organic compound contamination.) Silicone tubing is not recommended because it is gas permeable.
Section 2.3 Field Vehicles
Return to Section 2.1--Selection of Equipment for Water Sampling
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